1. Field of the Invention
The present invention is related to reformed gas production methods and reformed gas production apparatuses for reforming hydrocarbon fuels to produce a synthesis gas that includes hydrogen and carbon monoxide.
2. Description of Related Art
This synthesis gas can be produced by reforming hydrocarbons or methanol, for example, and a reformed gas production apparatus that produces synthesis gas through such fuel reforming can be employed in the fields of synthesis gas production, such as the production of liquid fuel from natural gas (GTL), and the production of hydrogen for fuel cells, for example.
Currently, the three methods of steam reforming, partial oxidation, and autothermal reforming, which combines these two methods, are the primary methods for reforming hydrocarbon fuels.
Steam reforming is primarily used in the field of hydrogen production because of the high H2/CO ratio in the synthesis gas that it yields through Equation 1 below, and is employed for many applications. Because hydrogen is created from not only the hydrocarbon but also from the steam, there is a high H2/CO ratio, making this method effective for the production of hydrogen. On the other hand, being an endothermic reaction, the addition of heat from the outside becomes the rate-limiting factor and prevents the gas-space velocity (gas flow/catalyst amount) from being made large.CnH2n+2+nH2O→(2n+1)H2+nCO  [Eq. 1]
Partial oxidation (also known as partial combustion) is an exothermic reaction in which, as shown by Equation 2 below, fuel is partially oxidized (partial combustion) to produce hydrogen. This method is advantageous in that the reaction rate is fast and in that a large gas-space velocity can be achieved, while on the other hand its low thermal efficiency is one drawback to this method.CnH2n+2+0.5nO2→(n+1)H2+nCO  [Eq. 2]
Partial oxidation yields a synthesis gas whose H2/CO ratio is lower than that of the synthesis gas produced by steam reforming, and is effective for instances where a low H2/CO ratio is preferable for the application in which the synthesis gas will be used. When fuel is reformed through partial oxidation, however, at the same time that hydrocarbons such as methane are reformed they may undergo dehydrogenation/oxidation and become heavy hydrocarbons or carbon, making continuous operation difficult and, as mentioned above, the thermal efficiency of partial oxidation is poor. Together these make it difficult for partial oxidation to be adopted as the method for reforming natural gas and naphtha, for example.
Autothermal reforming is a composite system in which the partial oxidation reaction and the steam reforming reaction are run in series or simultaneously to supply the heat required for the endothermic steam reforming reaction using the heat produced through partial oxidation. Generally there are two types of such systems. One is a series method of performing the partial oxidation reaction in the gas phase using a burner or the like and then performing steam reformation (hereinafter, abbreviated as ATR). The other is a method of running partial oxidation and steam reformation simultaneously using a catalyst (hereinafter, abbreviated as CPO). Methods for producing hydrogen or synthesis gas using these autothermal reforming methods have been disclosed in various publications (see JP 2000-84410A (pg. 2 to 14) and JP 2001-146406A (pg. 2 to 6)).
One problem with fuel reforming by an autothermal reforming method is that if the raw material is a hydrocarbon that has two or more carbon atoms, then there is much more pronounced carbon deposition near the inlet of the reformer than if methane is the raw material. That is, as shown in FIG. 17, the temperature near the inlet of the reformer rises abruptly due to the partial combustion of the hydrocarbon with oxygen, and this thermally decomposes hydrocarbons having two or more carbon atoms into carbon. For example, the working examples and comparative examples of JP 2001-146406A are examples in which carbon deposition was confirmed, and a conceivable reason for this is that the sudden elevation in the temperature of the catalyst layer inlet portion within the reaction chamber led to thermal decomposition of the hydrocarbon and subsequent carbon deposition.
In some instances, a pre-reformer is set up at an early stage as one way to solve the problem of fuel reforming by an autothermal reforming method (that being carbon deposition). Steam reformation and CPO are examples of reforming methods that may be performed at the pre-reformer (see JP 2002-97479A (pg. 2 to 5)). An autothermal reformer with pre-reformer such as that shown in FIG. 18 has been proposed as a steam reforming pre-reformer. That is, in the pre-reformer, the hydrocarbons are reformed into methane at a low temperature at which carbon deposition caused by steam reformation does not occur, and then the methane is reformed in an autothermal reformer and thereby converted into hydrogen and carbon monoxide. JP 2002-97479A discloses a CPO pre-reformer+ATR technology and mentions that all higher-order hydrocarbons are converted during CPO. It is clear that this technology is to be practiced under the premise that saturated hydrocarbons having two or more carbon atoms are converted by CPO.
However, this autothermal reformer with pre-reformer has the problem that if the pre-reformer is adopted for steam reforming, then the steam reforming reaction, whose rate of reaction is slower than that of the partial oxidation reaction, is performed at low temperatures, thus requiring large amounts of catalyst and therefore not allowing the reformer to be provided compact. Additionally, there is also the problem that if this autothermal reformer with pre-reformer is employed in the reforming system of an automobile fuel cell or a home fuel cell, in which starting and stopping is necessary on daily basis, then, because starting from normal temperatures is requires a significant amount of time, it becomes necessary to constantly burn hydrocarbon fuel, which is the raw material for reforming, in order to keep the reformer near the reforming temperature.
Adopting the pre-reformer for CPO allows the reforming efficiency to be increased, but on the other hand, the temperature of the catalyst layer inlet portion quickly becomes a high temperature, as discussed in JP 2001-146406A, and thus there is the problem that hydrocarbons having two or more carbon atoms are thermally decomposed and deposit carbon, precluding stable use of the apparatus over extended periods.